Polymethacrylimide foams have long been known and, owing to their excellent mechanical properties and their low weight, are widely used, in particular in the production of multilayer materials, laminates, composites or foamed composites. Prepregs comprising polymethacrylimide core materials are frequently bonded here.
For example, they are used in aircraft construction, in shipbuilding as well as in automotive construction. For many of these numerous applications, they have to meet technical requirements laid down in statutory provisions and a number of other regulations.
The present invention relates to the area of the polymer blocks produced by the casting method and polymethacrylimide foams prepared therefrom. Here, the monomers methacrylic acid and methacrylonitrile are introduced between two plane-parallel plates —generally glass plates. After the polymerization, the polymer sheets obtained are foamed in a further, separate method step.
The method relevant in production technology is based on foaming in a hot-air oven, which is to be referred to below as the hot-air method. The polymer sheets are introduced suspended in a forced-circulation oven, transported through said oven by a self-sustaining traction system and discharged at the end as foam sheets. The distance covered by the sheets in the oven is referred to below as L. The foaming time is thus defined by the length L of the oven and the constant travelling velocity V of the transport system in the total oven. The oven throughput depends not only on its length L and the travelling velocity V of the transport system but also on the time interval t and hence also the geometric spacing a of the sheets with which the latter are introduced into the oven. Since the sheets are greatly distorted during the foaming method, the spacing a must be larger than b/π so that the sheets cannot touch one another during the foaming and thus become damaged. b is defined as the length of the side from which the sheet is suspended and which the sheet has when it has been foamed. The content of this publication is limited to the method step comprising foaming.
DE 3 630 960 describes a further method for the foaming of the abovementioned copolymer sheets from methacrylic acid and methacrylonitrile. Here, the sheets are foamed with the aid of a microwave field, and this is therefore referred to below as the microwave method. It must be ensured here that the sheet to be foamed or at least its surface must be heated beforehand up to or above the softening point of the material. Since of course the foaming of the material softened by the external heating also begins under these conditions, the foaming method alone cannot be controlled by the influence of a microwave field but also must be controlled from the outside by accompanying heating. Thus, a microwave field is coupled with the usual one-stage hot-air method in order to accelerate the foaming. However, the microwave method has proved too complicated and therefore not relevant in practice and is not used today.
WO 90/2621 describes a foam obtained from methacrylic acid and methacrylonitrile, acrylamide as a comonomer preventing premature formation of precipitates during the polymerization. The foam formed is very uniform and the product has no internal stresses.
DE 197 17 483 describes a method for the preparation of polymethacrylimide foams to which 1–5% by weight, based on the monomer mixture, of MgO are added. Foams having substantially improved thermomechanical properties are obtained.
DE 196 06 530 describes the addition of a flameproofing agent by means of polymethacrylimide foams.